For 6 figure grid-references…

For 2 figure grid-references…

www.haroldstreet.org.uk/waypoints/download/?area=NN55-NJ55
gives the 100km square from the top right quarter of NN (i.e. from the middle at NN500500) to the bottom left right quarter of NJ (i.e. from the middle at NJ500500) including the bottom left corner of NH and the top right corner of NO. 2 figure grid references give a 10km resolution.